1. Field of the Invention
The invention relates to a flowmeter of simple structure which can effectively prevent waste material from entering a flow passage in which a flow sensor is arranged, without disturbing a flow of fluid, and/or effectively prevent aerosol (particles) contained in fluid from adhering to the surface of a thermal flow sensor, and to a smoking device using this flowmeter.
2. Description of the Related Art
A sensor for detecting the flow rate of fluid such as gas or air, there is known a thermal flow sensor having a structure in which a pair of temperature measuring parts (temperature detecting elements) are arranged in the direction of flow of fluid with a predetermined space between and a heat generating part (heater element) is arranged between these temperature measuring parts. The thermal flow sensor of this type makes use of the fact that the temperature distribution in the vicinity of the surface of the sensor produced when the heat generating part generates heat varies depending on the flow velocity of fluid. For example Japanese Unexamined Patent Publication No. 4-230808 discloses a thermal flow sensor which detects temperature difference as representing the above-mentioned temperature distribution using a pair of temperature measuring parts and determines the mass flow rate of fluid from the detected temperature difference. Japanese Unexamined Patent Publication No. 2-259527 discloses a thermal flow sensor in which a pair of temperature measuring parts are made to self-heat to detect the flow rate of fluid. In the thermal flow sensor of this type, the self-heating of the temperature measuring parts may cause a temperature-measurement error. Hence, generally, the accuracy of measurement is secured by keeping drive voltage for the temperature measuring parts as low as possible.
To solve a problem of smoke produced by smoking cigarettes, for example U.S. Pat. No. 5,878,752 discloses a smoking device which heats a cigarette with a heater when the cigarette is sucked on, to extract only its flavor and aroma without burning the cigarette. In this smoking device, the flow of air produced by sucking on a cigarette is detected with a thermal flow sensor of the above-described type, and the heater is driven to generate heat on the basis of the detection.
In this type of smoking device, when the flow of air (fluid) containing a large amount of various types of aerosol (particles) and/or waste material should be detected by a thermal flow sensor, there is a possibility that aerosol and/or waste material will adhere to and accumulate on the surface of the flow sensor. When aerosol and/or waste material adheres to the surface of the flow sensor, it leads to not only deterioration in measurement accuracy of the flow sensor but also breakage of the flow sensor. Hence, conventionally, a waste-material entry prevention member such as a filter or a rod-like protrusion is provided in the flow passage, upstream of the flow sensor.
However, when the waste-material entry prevention member is provided like this, the waste-material entry prevention member interferes with the flow of fluid, so that the flow rate cannot be measured accurately. Further, when the flow rate of fluid varies rapidly like when the fluid begins to flow, vortexes (for example Karman vortexes) tend to be produced in the flow of fluid after the waste-material entry prevention member. Hence, when the flow rate should be measured, for example it is necessary to obtain a measured value repeatedly over a predetermined period of time, determine whether the fluid has been flowing smoothly or not on the basis of the obtained measured values, and then evaluate those measured values. Specifically, it is necessary to determine, on the basis of a tendency found in variation of the measured value, whether or not temperature variation has been produced by fluctuation of the fluid flow or the like. Hence, it takes time before an accurate measured value is obtained, and deterioration in responsiveness is inevitable.
Further, even if the waste-material entry prevention member is provided, aerosol and/or waste material contained in gas or air gradually accumulates on the surface of the thermal flow sensor when the thermal flow sensor is used over a long period of time. The aerosol and/or waste material accumulated on the sensor surface prevents transmission of heat from the heat generating part and to the temperature measuring parts and thereby makes the measurement accuracy worse. The accumulated aerosol and/or waste material also produces problems such that it makes the speed of response in flow-rate detection worse. Particularly when the thermal flow sensor is used as a current-supply control means for a heater in the above-described smoking device, the above-mentioned problems are serious, since aerosol flowing from a cigarette as so-called side stream smoke easily accumulates on the surface of the thermal flow sensor.